Propylene and other lower olefins used in polymerization processes must be free of contaminants which poison the polymerization catalysts and/or otherwise distort the polymerization process. A contaminant which has received particular attention for propylene purification is carbonyl sulfide; since carbonyl sulfide tends to vary considerably within a range of small concentrations in propylene, it is particularly difficult to contend with and its removal has accordingly been the subject of a number of patents. See, for example, U.S. Pat. Nos. 4,613,724 and 4,444,987.
The arsenic compound of principal concern in propylene purification applications is most commonly arsine since it is the most volatile form of arsenic and also boils at a temperature close to the boiling point of propylene. However, other forms of arsenic may also be present and reference to arsenic herein is intended to include arsine and all other forms of arsenic that may be present in the process stream. The arsenic is removed by reacting or complexing with the metal oxide or oxides contained in the catalyst. The terms "removal" and "adsorption" of arsenic by the catalyst as used in this disclosure implies that such reacting or complexing occurs. Such complexing results in more complete removal of the arsenic contaminant than conventional adsorption processes would permit.
In U.S. Pat. No. 4,009,009 the use of a lead oxide catalyst on an alumina support is suggested for the removal of COS from propylene; in U.S. Pat. No. 3,782,076 a similar lead oxide containing catalyst is suggested for the removal of arsine from hydrocarbons. Arsine has been found to be a particularly unpredictable contaminant in both concentration and effect. The removal of arsine (Ash.sub.3) is attendant with additional difficulties, however, in that its infamous reputation as a poison requires that it be disposed of in a prudent manner. The process of U.S. Pat. No. 3,782,076 has not conclusively addressed this problem.
The present invention is an improvement on U.S. Pat. No. 3,782,076 wherein a lead oxide catalyst on an alumina support is used to remove arsenic compounds from hydrocarbon streams. The present invention is a practical and efficient method of activating lead oxide catalysts and other supported metal oxide catalysts having been used to remove arsine and/or other arsenic compounds from hydrocarbon steams, particularly lower olefins and, more particularly, propylene.
U.S. Pat. No. 3,812,652 by Carr suggests regenerating the spent arsenic removal catalyst by contact with a gas stream containing free molecular oxygen. Air is cited as an example of such a gas stream. However, in some process applications such as propylene polymerization, the use of oxygen in the system in highly undesirable. Therefore, to apply the teachings of U.S. Pat. No. 3,812,652, the guard bed catalyst must be removed from the system and processed externally. U.S. Pat. No. 4,593,148 also teaches the use of air for catalyst regeneration and thereby suffers the same disadvantage as U.S. Pat. No. 3,812,652.
The reader may also be interested in U.S. Pat. Nos. 4,593,148, 4,605,812, 4,442,077, 4,088,734, 4,044,067, 4,150,063, and 4,613,724, all of which remove either COS or arsine, or both, from hydrocarbons. Generally, the catalysts are either not regenerated or are regenerated with oxygen or air. The regeneration process tends to be somewhat complicated by the fact that in most of the cases, the removal of arsine takes place along with the removal of sulfur compounds such as carbonyl sulfide. Thus, regeneration of the catalyst tends to be dominated by the process of removing the sulfur, and the arsenic, being present in generally smaller quantities, is necessarily subject to the same regenerative process, which may not be particularly advantageous for arsenic removal. An additional complication is the fact that lead oxide reacts irreversibly, for practical purposes, with sulfur--thus, where a lead oxide catalyst is used on a stream which contains both arsine and sulfur-containing compounds, it suffers from the dual problem that arsine pickup may be interfered with and there may be a drastic diminishing of the ability to be regenerated. It should be noted that the inventors in U.S. Pat. No. 3,782,076 recognize (col. 3, lines 16-28) that arsine removal is more efficient when sulfur compounds have been previously removed from the subject gas.
We have developed a novel process for the purification of hydrocarbon steams which includes a new process for the rejuvenation of a supported metal oxide or mixed oxide catalyst loaded with arsenic.
The experimentation performed in support of this disclosure suggest that the total capacity of the supported oxide catalyst for arsenic removal may be is limited by the catalyst composition and the stiochiometry of the reaction. Since in our procedure we do not actually remove arsenic from the combined catalyst and its support, but enable it to achieve its maximum loading, the regeneration of the catalyst which we accomplish is in reality a rejuvenation and is referred to as such herein. In most instances, it is economically advisable to rejuvenate the supported metal oxide or mixed oxide catalyst rather than to replace it. It is also advantageous to perform this rejuvenation in situ rather than incurring the additional expense and down time associated with emptying the reaction vessel and performing the procedure off-line. Disclosure of the Invention
The supported metal oxides or mixed oxides to which this disclosure applies are those which complex or react with arsenic compounds and particularly arsine at temperatures below 300.degree. C. and preferably below 100.degree. C. The literature reports that copper oxide, copper chromite, copper oxide-zinc oxide, and lead oxide catalysts will remove arsenic compounds from hydrocarbon feed; see U.S. Pat. Nos. 3,782,076, 3,789,581, 3,812,652, 4,605,812, and 4,593,148. The reference cited suggest that the catalysts be operated at from 80.degree. to 150.degree. F.
Our invention includes rejuvenation of arsenic-containing catalysts of the above description and/or any other oxide or mixed oxides used for this purpose. The basic utility of this invention is that the procedures specified do not admit any toxic materials to the atmosphere and thereby have no environmental impact. The procedures specified require no air or oxygen and are therefore safe to practice in explosive atmospheres. These procedures may be performed on the catalysts in situ thereby eliminating the lost operating time and expense which accompanies any catalyst treatment performed off-line. Lastly, this invention greatly extends the catalyst life.
We have found that spent supported metal oxide catalysts used for arsine removal may be rejuvenated [provided the amount of arsenic in the catalyst has not reached the adsorption limit of the catalyst] by heating same in an inert gas stream or in an inert atmosphere at a temperature from 50.degree. to 400.degree. C. and preferably from 100.degree. to 400.degree. C. for a period of at least about 1/2 hour and preferably from 1 to 100 hours. Processing beyond this time will generally not improve results commensurate with the additional effort required. Said catalysts may also be rejuvenated by heating same in a stream of wet inert gas at a GHSV of at least about 100 and at a temperature from 50.degree. to 400.degree. C. and preferably from 100.degree. to 400.degree. C. for a period of at least about 1/2 hour and preferably from 1 to 50 hours with the moisture content of the inert gas is being from 0.001 to 80 mol percent and preferably from 0.01 to 20 mol percent. Processing beyond this time will generally not improve results commensurate with the additional effort required. Those skilled in the art of process engineering can adjust moisture content of the inert gas stream in correspondence with the process conditions used to accomplish the desired extent of catalyst rejuvenation. Said catalysts may also be rejuvenated by heating same in a steam environment at from 100.degree. to 400.degree. C. for a period of at least about 1/2 hour and preferably from 1 to 50 hours. Processing beyond this time will generally not improve results commensurate with the additional effort required. Said catalysts may be rejuvenated by adding from 0.1 to 50% moisture and preferably from 1 to 15% moisture to the catalyst and heating same in an inert gas stream or in an inert gas atmosphere at from 50.degree. to 400.degree. C. and preferably from 100.degree. to 400.degree. C. for a period of at least about 1/2 hour. Again, while processing in excess of 50 hours is not harmful, little if any additional benefit may be expected by doing so.
In one aspect, our invention is a process for the rejuvenation of a supported metal oxide such as a lead oxide catalyst having been used for the adsorbtion of arsenic, wherein the loaded catalyst is treated by one of the procedures described above. The catalyst should be on a porous base or support, preferably, but not limited to, an inorganic oxide material substantially comprising either one metal oxide or a mixture of oxides, carbides, silicates or aluminates; in either case, the type of support commonly used in preparing catalysts and familiar to those skilled in the art such as, but not limited to, alumina, silica, silica-alumina, magnesia, silica magnesia, calcium aluminates, calcium silicates, zinc oxide and silicon carbide. In another aspect, our invention is a sequence of purification steps for propylene and other lower olefins wherein the lower olefin is passed through a catalyst bed specifically adapted for the removal of carbonyl sulfide and subsequently passed through a bed of supported lead oxide catalyst for the removal of arsine. The arsine removal catalyst bed, free of sulfur, is then subjected to one of the rejuvenation treatments described above.